Most switching mode power supplies operate at a fixed and fast switching frequency. Unfortunately, a strong noise would be generated at such switching frequency and its relative harmonic, thereby inducing loud electromagnetic interference (EMI). Jittering or dithering switching frequency can distribute the noise's energy to a broad band, and thus depresses the noise and improves the performance in conduction of EMI. As is well known in the art, the switching frequency of a switching mode power supply depends on the clock frequency of the oscillator in the power supply. For further detail, FIG. 1 shows a conventional oscillator 100, in which a comparator 102 will trigger a comparison signal COMP1 to a set input S of a flip-flop 110 when the voltage Vosc on the capacitor C-osc is lower than a lower limit VREF1 of a hysteretic band, causing the output Q of the flip-flop 110 to be high, and thereby turning off a switch S4 for a current source 108 to stop discharging the capacitor C_osc, and turning on a switch S3 for a current source 106 to charge the capacitor C_osc. When the voltage Vosc increases to touch an upper limit VREF2 of the hysteretic band, a comparator 104 triggers a comparison signal COMP2 to a reset input R of the flip-flop 110, causing the output Q of the flip-flop 110 to be low, and thereby turning off the switch S3 for current source 106 to stop charging the capacitor C_osc, and turning on the switch S4 for the current source 108 to discharge the capacitor C_osc. A one shot circuit 112 generates a clock CLK according to the output Q of the flip-flop 110. As shown in the oscillator 100 of FIG. 1, the frequency of the clock CLK is determined by the charging current IREF 1, the discharging current IREF2, the capacitance C_osc and the hysteretic band corresponding to the lower limit VREF1 and the upper limit VREF2. U.S. Pat. No. 6,249,876 to Balakrishnan et al. disclosed a frequency jittering control for varying the switching frequency of a power supply by adjusting the charging and discharging currents in the oscillator circuit of the power supply, and thus reduces the EMI emission. However, this art requires a huge capacitor to be charged and discharged, which is disadvantageous to cost reduction and chip circuit integration.
Therefore, it is desired a smaller and cost-effective frequency jittering circuit for varying the switching frequency of a power supply.